How to use Database as slow_backend instead of Files in Magento EE 1.12?

Question

In Magento EE 1.12.0.0 It would seem that no matter what configuration changes I make to app/etc/local.xml, the default file cache continues to be used (which is evidenced by var/cache/ always filling).

Expectation

• Memcached is used as fast_backend.
• Database is used as slow_backend.
• File cache is not used at all (i.e. var/cache/ should always be empty).

Actual Output

• Memcached is used as fast_backend.
• Database is not used at all.
• File cache is being used.

Testing Procedure

1. Make configuration change to app/etc/local.xml.
2. Restart Memcached and Apache (just for good measure and it's on my local dev box so I may as well).
3. Clear the file cache (rm -rf var/cache/*).
4. Refresh the home page.
5. Check the contents of the file cache (ls var/cache).
6. Become saddened and return to #1 with a different config change.

The Config

The contents of my app/etc/local.xml is as follows:

<config>
    <global>
        <install>
            <date><![CDATA[{{actual_data}}]]></date>
        </install>
        <crypt>
            <key><![CDATA[{{actual_data}}]]></key>
        </crypt>
        <disable_local_modules>false</disable_local_modules>
        <resources>
            <db>
                <table_prefix><![CDATA[]]></table_prefix>
            </db>
            <default_setup>
                <connection>
                    <host><![CDATA[{{actual_data}}]]></host>
                    <username><![CDATA[{{actual_data}}]]></username>
                    <password><![CDATA[{{actual_data}}]]></password>
                    <dbname><![CDATA[{{actual_data}}]]></dbname>
                    <initStatements><![CDATA[SET NAMES utf8]]></initStatements>
                    <model><![CDATA[mysql4]]></model>
                    <type><![CDATA[pdo_mysql]]></type>
                    <pdoType><![CDATA[]]></pdoType>
                    <active>1</active>
                </connection>
            </default_setup>
        </resources>
        <session_save><![CDATA[db]]></session_save>
        <cache>memcached</cache>
        <slow_backend>database</slow_backend>
        <slow_backend_store_data>1</slow_backend_store_data>
        <memcached>
            <servers>
                <server>
                    <host><![CDATA[{{actual_data}}]]></host>
                    <port><![CDATA[{{actual_data}}]]></port>
                    <persistent><![CDATA[0]]></persistent>
                    <weight><![CDATA[2]]></weight>
                    <timeout><![CDATA[10]]></timeout>
                    <retry_interval><![CDATA[10]]></retry_interval>
                    <status><![CDATA[]]></status>
                </server>
            </servers>
            <compression><![CDATA[0]]></compression>
            <cache_dir><![CDATA[]]></cache_dir>
            <hashed_directory_level><![CDATA[]]></hashed_directory_level>
            <hashed_directory_umask><![CDATA[]]></hashed_directory_umask>
            <file_name_prefix><![CDATA[]]></file_name_prefix>
        </memcached>
    </global>
    <admin>
        <routers>
            <adminhtml>
                <args>
                    <frontName><![CDATA[admin]]></frontName>
                </args>
            </adminhtml>
        </routers>
    </admin>
</config>

Answer 1

I think that is not the right format for the cache nodes. My understanding is that all the cache settings should be nested inside the <cache> node. So to use two-level cache with memcached + database it would be something like this:

<cache>
    <backend>memcached</backend>
    <slow_backend>database</slow_backend>
    <memcached>
        <servers>
            <server1>
                <host>...</host>
                <port>11211</port>
                <persistent>1</persistent>
                <weight>2</weight>
                <timeout>10</timeout>
                <retry_interval>10</retry_interval>
                <status/>
            </server1>
            ...
        </servers>
        <compression>0</compression>
        <cache_dir/>
        <hashed_directory_level/>
        <hashed_directory_umask/>
        <file_name_prefix/>
    </memcached>
</cache>

Bear in mind that <full_page_cache> can be configured exactly the same way and can use different settings if you wish. They are just two separate cache instances.

Just as a side note, I would highly recommend using Redis instead. It supports tags, so it can be used as a single level cache and it will perform much better than two-level memcached + database.